Our previous work revealed that psychomotor stimulant drugs such as MDMA and methamphetamine induce robust changes in brain temperature. These hyperthermic effects are strongly potentiated by social interaction and warm environment--two conditions often associated with human drug use. In this year, we worked with heroin, another abused drug, which could induce lethality during overdose. We also initiated work with fentanyl, a potent opioid drug, which is often contained in street heroin and could make it much more dangerous. We showed that heroin even at low, self-administering doses induces moderate brain and body hyperthermia, which is underlined by strong skin vasoconstriction and weak brain metabolic activation. We also showed that this pattern of temperature and metabolic response is changed qualitatively when the dose of heroin is increased. Under these conditions, heroin induced prolonged decreases in brain and body temperature, which could be related to comatose-like state. We also examined temperature effects of fentanyl, which has distinct differences from heroin.